The first approach to employing a light sensitive inhibitor to probe an enzyme target was reported by Westheimer in 1962. From this one report, the application of photosensitive chemical probes to study biological systems has evolved to a state of rapid growth and diverse interest. More than 60 publications have appeared within the last 12 months dealing with various aspects of the subject. This area of investigation is quite young and contains many topics where increased understanding and formalization of problems is neccessary. In order to provide a comprehensive discussion of relevant topics we believe the conference should integrate symposia at each of the following levels of investigation: (1) the photochemistry underlying the generation of reactive transients and their subsequent fate. (2) The interaction of photoaffinity inhibitors with enzyme systems. (3) The application of photoaffinity probes to label and characterize biological receptor sites. (4) Studies of erythocytes, bacteria and subcellular organelles. (5) Studies on more complex whole-cell systems. Organized in this way, the conference will promote an exchange of information between investigators actively engaged in various aspects of photochemical probing of biological systems, as well as providing a sound introduction for those not presently familiar with the technique. A conference at this time will permit an exchange of ideas and discussion of controversial topics, and should contribute to successful growth of the field.